In a laser processing machine having a CO2 laser, the laser radiation for processing the material is generated by molecular vibrations. The generated laser radiation is usually guided through a gas atmosphere which, by appropriate measures, is kept free of substances that are capable of absorbing the generated laser radiation.
CO2 laser radiation can be absorbed by various molecules. A precondition for absorption is that one of the molecular bonds has the appropriate binding energy. Examples of such gaseous substances that should be kept away from the beam are SF6, C2H4, halogenated hydrocarbons, ammonia, alcohols, acetone, and CO2.
At a general level, the detrimental effect of those gases resides in the absorption itself and in an associated attenuation of the laser radiation power required for processing. Moreover, the optical effects on the laser radiation resulting from the absorption can include widening of the beam and a distortion of the beam phase front. An increase in temperature and the ensuing change in the refractive index of the medium through which the beam passes also can have an adverse effect on the laser radiation.
Studies have shown that an impurity of less than 100 ppb (0.1 ppm) SF6 is sufficient to cause impairment of the cutting of a sheet of steel at 3 kW laser power. SF6, with a wavelength of 10 μm, exhibits very high beam absorption.